Olefin polymerization catalyst systems can be classified into Ziegler-Natta and metallocene catalyst systems, and these two highly active catalyst systems have been developed in compliance with their characteristics. The Ziegler-Natta catalyst has been widely applied to existing commercial processes since it was developed in the 1950. However, since the Ziegler-Natta catalyst is a multi-active site catalyst in which a plurality of active sites are mixed, it has a feature that molecular weight distribution of polymers is broad. Also, since compositional distribution of comonomers is not uniform, there is a problem that it has a limitation in securing the desired physical properties.
Meanwhile, the metallocene catalyst includes a combination of a main catalyst whose main component is a transition metal compound, and an organometallic compound cocatalyst whose main component is aluminum. Such a catalyst is a single-site catalyst which is a homogeneous complex catalyst, and offers a polymer having a narrow molecular weight distribution and a uniform composition distribution of comonomers, depending on the single site characteristics. The metallocene catalyst has a feature such that it is capable of changing the stereoregularity, copolymerization properties, molecular weight, degree of crystallinity, and the like of the polymer by changing the ligand structure of the catalyst and the polymerization conditions.
U.S. Pat. No. 5,914,289 discloses a method of controlling the molecular weight and the molecular weight distribution of polymers using metallocene catalysts which are supported on respective supports. However, a large amount of solvent and a long period of time are required to prepare the supported catalysts, and the process of supporting metallocene catalysts on the respective supports is troublesome.
Korean Patent Application No. 10-2003-12308 discloses a method of controlling the molecular weight distribution of polymers by supporting a bi-nuclear metallocene catalyst and a mononuclear metallocene catalyst on a support with an activator, thus changing a combination of catalysts in a reactor and performing polymerization. However, this method has a limitation in simultaneously achieving the properties of the respective catalysts. In addition, there is a disadvantage that a metallocene catalyst portion is liberated from a supported component of the prepared catalyst to cause fouling in the reactor.
Therefore, in order to solve the above-mentioned disadvantages, there is a continuing need to develop a method for preparing olefinic polymers with the desired physical properties by easily preparing a supported hybrid metallocene catalyst having excellent activity.
On the other hand, a linear low density polyethylene is produced by copolymerizing ethylene and alpha olefins using a polymerization catalyst under low pressure. Thus, this is a resin having a narrow molecular weight distribution and a certain length of a short chain branch, without having a long chain branch. The linear low density polyethylene film has high strength and elongation at break in addition to the properties of a general polyethylene, and exhibits excellent tear strength, falling weight impact strength, and the like. This has led to an increase in the use of a stretch film, overlapping films, or the like to which it is difficult to apply existing low density polyethylene or high density polyethylene.
However, a linear low density polyethylene using 1-butene or 1-hexene as a comonomer is mostly prepared in a single gas phase reactor or a single loop slurry reactor, and has high productivity compared to a process using a 1-octene comonomer. However, these products have limitations in the catalyst technology and process technology. Thus, they are problematic in that their physical properties are greatly inferior to when using a 1-octene comonomer, and the molecular weight distribution is narrow, resulting in poor processability. Many studies have been conducted to ameliorate these problems.
U.S. Pat. No. 4,935,474 describes a process for preparing polyethylene having a broad molecular weight distribution by using two or more metallocene compounds. U.S. Pat. No. 6,828,394 discloses a process for producing polyethylene which is excellent in processability and particularly suitable for a film, by using a catalyst system including a poor comonomer incorporating catalyst compound and a good comonomer incorporating catalyst compound. In addition, U.S. Pat. Nos. 6,841,631 and 6,894,128 describe that polyethylene having a bimodal or multimodal molecular weight distribution is produced by using a metallocene-type catalyst including at least two metal compounds, and thus it can be used in various applications such as films, blow molding, and pipes. However, although these products have improved processability, there is still a problem that the dispersed state per molecular weight within unit particles is not uniform, and thus even under relatively good extrusion conditions, the extruded appearance is rough and the physical properties are not stable.
Given the above factors, there is a continuing demand for the production of superior products having a balance between physical properties and processability, and improvement thereof is still necessary.